Datasheet
33
2545T–AVR–05/11
ATmega48/88/168
9.5 Low frequency crystal oscillator
The device can utilize a 32.768kHz watch crystal as clock source by a dedicated low frequency
crystal oscillator. The crystal should be connected as shown in Figure 9-2 on page 30. When this
oscillator is selected, start-up times are determined by the SUT fuses and CKSEL0 as shown in
Table 9-7.
Note: 1. These options should only be used if frequency stability at start-up is not important for the
application.
9.6 Calibrated internal RC oscillator
By default, the internal RC oscillator provides an approximate 8.0MHz clock. Though voltage
and temperature dependent, this clock can be very accurately calibrated by the user. The device
is shipped with the CKDIV8 fuse programmed. See “System clock prescaler” on page 36 for
more details.
This clock may be selected as the system clock by programming the CKSEL fuses as shown in
Table 9-8 on page 34. If selected, it will operate with no external components. During reset,
hardware loads the pre-programmed calibration value into the OSCCAL Register and thereby
automatically calibrates the RC Oscillator. The accuracy of this calibration is shown as Factory
calibration in Table 29-1 on page 306.
By changing the OSCCAL register from SW, see “OSCCAL – Oscillator calibration register” on
page 37, it is possible to get a higher calibration accuracy than by using the factory calibration.
The accuracy of this calibration is shown as user calibration in Table 29-1 on page 306.
When this oscillator is used as the chip clock, the Watchdog Oscillator will still be used for the
watchdog timer and for the reset time-out. For more information on the pre-programmed calibra-
tion value, see the section “Calibration byte” on page 288.
Table 9-7. Start-up times for the low frequency crystal oscillator clock selection.
Power conditions
Start-up time from
power-down and
power-save
Additional delay
from reset
(V
CC
= 5.0V) CKSEL0 SUT1..0
BOD enabled 1KCK 14CK
(1)
000
Fast rising power 1KCK 14CK + 4.1ms
(1)
001
Slowly rising power 1KCK 14CK + 65ms
(1)
010
Reserved 0 11
BOD enabled 32KCK 14CK 1 00
Fast rising power 32KCK 14CK + 4.1ms 1 01
Slowly rising power 32KCK 14CK + 65ms 1 10
Reserved 1 11